1. FIELD OF THE INVENTION
The present invention relates to electroviscous fluids.
2. DESCRIPTION OF THE PRIOR ART
It is known that certain suspensions consisting of finely devided particles dispersed in a highly non-conductive liquid exhibit a remarkable increase in viscosity in an applied electric field. This effect is termed the Winslow Effect and these suspensions are termed electroviscous fluid of Winslow type. Most electroviscous fluids proposed heretofore comprise an electrically insulating liquid and, dispersed therein, a quantity of water-containing fine particles and are of Winslow type. According to two leading theories on the mechanism by which the electroviscous effect occurs, the dielectric polarization and the hydrogen bonding due to water existing on the surfaces of particles form bridges of the particles in an applied electric field. As the methods of increasing the electroviscous effect, there are proposed many types of fine particles useful for electroviscous fluids including, for example, particles obtained by incorporating an aqueous solution of a metal ion or a polar substance in between the layers of a substance having a lamination layer structure such as mica and vermiculite [Japanese Patent Publication (Kokoku) No. 5117/1974], water-containing particles of strongly acidic or strongly basic ion exchangers [Japanese Patent Publication (Kokai) No. 92278/1975], water-containing particles of a high water-absorptive resin having an acidic group such as polyacrylic acid (U.S. Pat. No. 4,129,513), particles of pyrogenic silica in which an acid such as formic acid, maleic acid and a base such as aniline, ethylenediamine, easily capable of forming a hydrogen bond, have been incorporated instead of water (U.S. Pat. No. 3,427,247), as particles containing no water, particles of a ferroelectric substance such as potassium titanate [J. Appl. Phys. 38 (1) 67 (1967) ], and particles of an organic semiconductor such as lamp black [J. Appl. Phys. 21, 402 (1950)] and poly(acene-quinone)polymer (U.S. Pat. No. 4,687,589). However, the electroviscous fluids comprising the above described particles have some problems. More specifically, according to the electro-viscous fluids using water-containing particles, due to the presence of water, the water migrates into both the inside of the particles and the vehicle of the particles or water is vaporized or electrolyzed and the current generated is rapidly increased with elevated temperatures. According to the electroviscous fluids using particles of an acid and a base and particles of a semiconducor, the current is rapidly increased with elevated temperature. Further, according to the electroviscous fluids using particles of a ferroelectric substance, the electroviscous effect is low.
In general, the electroviscous fluids proposed heretofore exhibit the electroviscous effect by the application of an electric potential with either an alternating current or a direct current but when an electric potential is continuously applied for a long period of time, such a tendency that the electric potential is slowly decreased or dielectric breakdown easily occurs can be observed. In order to prevent the tendency, the use of an electric potential with a pulsating direct current is proposed (U.K. Patent No. 2125230).